Surface Segregation Anisotropy and the Equilibrium Shape of Alloy Crystals

摘要

A model of anisotropy of surface energy, resulting from surface segregation, has been used to calculate the equilibrium crystal shape (ECS) of FCC alloy crystals. The parameters of the model, including (a) surface energy differences between pure solvent and solute, (b) the regular solution constant, and (c) solute strain energy, have been varied in order to evaluate their effects on the ECS. Whereas the model predicts that the ECS of pure metals should only display {111} and {100} facets, the effects of segregation can produce several new facets, e.g. at {110}, {311}, {331}, and {210} orientations. In particular, new cusps in the g-plot can only appear if the solute strain energy parameter has finite values. Also, it has been found that for any pair of orientations, the ratio of surface energies displays a minimum at the temperature corresponding to the crossover in adsorption for those two surface orientations. These crossovers occur over a limited temperature range, and new cusps that are deep enough to produce new facets on the ECS also tend to occur in that restricted temperature range. Some comparisons with experimentally determined ECSs of alloy crystals are provided, and show that the model can provide valuable though qualitative guidance for experiments.